Typically, a self-expanding stent built up of a stack of zig-zag stenting rings is released from its catheter delivery system by proximal withdrawal of a surrounding sheath relative to the stent. Typically, there is within the catheter delivery system a form of a “stopper” that restrains the stent from moving proximally with the sheath, during the time when the sheath is being pulled back proximally.
In consequence, the distal end annulus of the sheath slides proximally over the full length of the stent, from the distal end of the stent to the proximal end of the stent, in order to release the stent into the body at the stenting site.
The stent being self-expanding, it is pressing on the luminal surface of the surrounding sheath. Stent portions escaping from the restraint of the sheath, as the distal end of the sheath slides proximally past them, will spring radially outwardly. The portion of the stent that is escaping from confinement in the sheath, continues to expand radially outwardly with increasing distance from the retreating end annulus of the sheath, until it presses against the tissue defining the bodily lumen to be stented at the stenting site.
When the stent is composed of a stack of zig-zag stenting rings, with only a few links connecting adjacent stenting rings, the end annulus of the retreating sheath will experience a cyclically varying hoop stress and pattern of radially outward pressure from the matrix of the stent confined within the sheath. Specifically, the forces imposed by the stent on the distal end annulus of the sheath tend to vary cyclically, as each one of the sequence of stenting rings passes the open distal end of the sheath. WO 2004/032802 A2 and EP 1 767 240 A1 address this behaviour.
FIGS. 1a and 1b of WO 2004/032802 A2 show a stent matrix formed of a plurality of interconnected stenting rings. The stenting rings are formed of repeating units of six struts arranged in V-shaped pairs between the links by which adjacent stenting rings are connected. At one axial end of each ring, the points of inflection between the struts in each pair are obliquely staggered in one direction relative to the stent longitudinal axis. Links are formed on the middle ones of these staggered groups of three pairs. The end struts in the repeating units are at one circumferential side of the unit much longer and at the other circumferential side of the unit much shorter than the other four struts in the repeating unit. The long and short struts connect to an adjacent stenting ring at the opposite axial end of the stenting ring from the middle links. Stenting rings joined by the middle links have the obliquely staggered inflection points facing each other, so that the oblique staggering between the facing ends matches; whilst the opposite ends are not obliquely staggered, but are aligned with similar non-regularly staggered points of inflection on the facing ends of further adjacent stenting rings. There are no substantial gaps between the ends of adjacent stenting rings, and only two points of inflection between links on the same end of each ring. There are multiple links between each pair of adjacent stenting rings, around the circumference of the stent.
EP 1 767 240 A1 discloses a stent formed of connected stenting rings. The points of inflection at each end of each ring are staggered alternately, at only two longitudinal positions at each end, between each two links, although there are four such points of inflection between each two links. The struts of the stenting rings are not substantially equal in length, but vary between three different lengths to produce the two-level longitudinal staggering on both ends of the stenting ring.
The behaviour of such stents can be contrasted with the situation where the same delivery system is used to deploy a self-expanding stent that is based not on a stack of stenting rings but on a continuous helical stack pattern of strut end points of inflection. In such a case, given a smooth and stepless withdrawal of the sheath, there will be a steady succession of escapes of points of inflection from the distal end of the sheath, with no two points of inflection escaping at precisely the same moment. It can be appreciated that such a helical stent is not likely to exhibit any significant tendency to “jump” out of the open distal end of the sheath. Conversely, a stent that exhibits a “stenting ring” format has a greater tendency to emerge from the proximally retreating sheath in a series of small jumps (and maybe with one bigger jump as the proximal end of the stent finally escapes from the distal end of the sheath).
It is one object of the present invention to improve the release behaviour of a series of stenting rings from a retreating sheath.
Often, a stent which is in the nature of a series of axially spaced endless stenting rings will have a greater capability to push bodily tissue radially outwards than will a stent that exhibits the form of a continuous helix or spiral from one end of the stent to the other. This greater capability to push radially outwardly of course can raise the amount of force imposed by the stent on the distal end annulus of the retreating sheath and thereby increase the tendency of such a stent to jump distally away from the retreating sheath.